System with inductively coupled temperature sensing units



R. B. JONES SYSTEM WITHU INDUCTIVELY COUPLED TEMPERATURE SENSING UNITSFiled Sept. 24, 1988 Nov. 10, 1970 2 Sheets-Sheet 1 1 M1- iimji .8

flw .QIW 6F IIIIN I 1 I I 1 l I I I 1 1 I i 1 1 I I 1 g a i i i E k VT\Ll/lllsl=|\| w IMNPXIIL Nov. 10, 1970- JONES 3,540,024

SYSTEM WITH INDUCTIVELY COUPLED TEMPERATURE SENSING UNITS Filed Sept.24, 1968 2 Sheets-Sheet 2 INVENTOR ROGER B. J ONES United States PatentUS. Cl. 340-228 6 Claims ABSTRACT OF THE DISCLOSURE A temperaturesensing system including a thermistor element composed of inductivelycoupled sections joined end to end, and a circuit connected to one endof the element for detecting a change in the resistance of thethermistor material by monitoring the impedance reflected back to thecircuit through the inductive couplings.

BACKGROUND OF THE INVENTION Field of the invention The present inventionrelates to apparatus for detecting abnormal temperature conditions, and,more particularly, to an arrangement which eliminates the effects of thepresence of moisture in the connecting device joining sensing elementsections.

The present invention, although useful for many purposes, is primarilyconcerned with utilizing an elongated heat detecting element comprisingtwo conductors within a metallic sheath and spaced apart by a thermistormaterial which has a negative temperature coefficient of resistivity anddecreases in resistance to a predetermined value in the presence of atemperature condition to be detected. Circuitry is provided formonitoring the resistance present between the wires and for giving analarm when this resistance drops to the predetermined value.

These detecting elements are normally made in sections to facilitateinstallation, particularly in cramped 1ocations, through bulkheads, etc.

Description of the prior art In the elements currently in use, eachsection is sealed at the ends to prevent atmospheric moisture fromgaining access to the thermistor material separating the conductors.However, the ends of the conductors are connected to electrical contactswhich pass through the seals and are part of connector members formed onthe ends of the section. These electrical contacts mate with thecontacts in the connector members of the adjoining element sections. Themating connector members are held together by a threaded member. Thisarrangement is shown in detail in US. Pat. 3,150,311.

It has been found that the connector formed by the adjoining of twomating connector members cannot be maintained air tight, and, therefore,moisture present in the atmosphere enters the connectors andcontaminates the contacts and the inner surfaces of the connectors toprovide current paths between the contacts in parallel with thethermistor material between the conductors. Such current paths arefrequently of very low resistance and therefore are capable of causingflie resistance monitoring circuitry to give a false indication that thepredetermined temperature condition exists. The presence of moisturewithin the connectors also causes electrolytic corrosion of thecontacts.

In the systems previously in use, attempts have been made to minimizethe low resistance effect of moisture in the connectors by decreasingthe resistance of the thermistor material at the predeterminedtemperature, in

order to make this value lower than the resistance of the moisturecurrent path, and also by increasing the resistance of the moisturecurrent path, for example, by using a direct current potential topolarize the conductive solution. Although these approaches tend toreduce the resistance effect of moisture they do not eliminate it, andthey have no effect at all upon the electrolytic corrosion.

SUMMARY An object of the present invention is to provide an improvedtemperature detecting apparatus not affected by the presence of moisturein its element connectors.

Another object is to provide such apparatus in which the detectingelement sections have no electrical contacts subject to being bridged bymoisture.

Another object is to provide such apparatus in which the detectingelement sections are inductively coupled.

Other and further objects of the invention will be obvious upon anunderstanding of the illustrative embodiment about to be described, orwill be indicated in the appended claims, and various advantages notreferred to herein will occur to one skilled in the art upon employmentof the invention in practice.

In accordance with the present invention, the foregoing objects aregenerally accomplished by providing a temperature detecting systemincluding first and second elongated elements each having first andsecond ends and comprising a pair of parallel spaced electricalconductors extending between the ends, at least the second element alsoincluding a mass of material between and in electrical contact with theconductors thereof, the material having an electrical resistance whichchanges in value in response to temperature changes to alter theresistance between the conductors, means for holding the first end ofthe second section closely adjacent to the second end of the firstsection, means in the first end of the second section and in the secondend of the first section for inductively coupling the conductors in thefirst section to the conductors in the second section, a source ofvarying potential electrical power coupled to the first end of the firstsection to be transmitted through the inductive coupling means into thesecond section, means for detecting a change in the resistance of thematerial by monitoring the impedance appearing at a first end of thefirst section, and means for giving an indication when the impedancereaches a predetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of theinvention has been chosen for the purpose of illustration anddescription, and is shown in the accompanying drawings, forming a partof the specification, wherein:

FIG. 1 is a schematic diagram of the system of the present inventionwherein the thermistor material between the conductors is represented bya number of parallel variable resistances.

FIG. 2 is a longitudinal sectional view of a portion of a detectingelement showing the construction of the connector inductively couplingthe element sections.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of thedrawings in detail there is shown, in schematic form, a fire detectingsystem according to the present invention which includes a firedetecting element 10 comprising a plurality of sections 11 positionedend to end, a circuit 12 for monitoring the condi tion of the element10, and a cable 13 for interconnecting the element 10 with the circuit12.

Each of the detecting element sections 11 as shown in FIGS. 1 and 2include a pair of parallel conductors 14 and embedded in a mass 16 (FIG.2) of material having a negative coefficient of resistance, representedby the variable resistances 16" in FIG. 1. Transformer assemblies 17 and18 are positioned at opposite ends of each of the element sections 11.Each of these assemblies include onehalf of a transformer having awinding 19 electrically connected across the ends of the conductors 15and 16, and a section 20 of a transformer core. As shown in FIG. 2, eachof the sections 11 is provided with a metal sheath 21 which extends thefull length of the section.

The cable 13 includes a pair of conductors 22 and 24 embedded in aninsulating ceramic material and in electrical connection with atransformer assembly 25 provided on the end of the cable connected tothe element 10. The assembly 25 is identical to the assemblies 18 of theelement sections 11 described in detail hereinafter and includes awinding 19 and a core section 20.

The circuit 12 includes a source of alternating current power 26, atransformer 27, an impedance measuring bridge 29, a full wave rectifyingbridge 30, a relay 31, and an indicator light 32.

The transformer 27 has an input winding 34 connected to the source and apair of output windings 35 and 36. The impedance measuring bridge 29 hasa pair of input terminals 37, 39 connected to the transformer winding35, a pair of output terminals 40, 41 connected to the rectifying bridge30, an impedance device 42 connected between the terminals 37 and 40, animpedance device 44 connected between the terminals and 39, and animpedance device 45 connected between the terminals 39 and 41. Theconductors 22 and 24 are connected to the terminals 37 and '41respectively to make the impedance of the element 10 the fourth leg ofthe impedance measuring bridge.

The rectifying bridge 30 includes four diodes 46 arranged inconventional manner between a pair of input terminals 47 and 49, whichare connected to the output terminals 40 and 41 of the bridge 29, and apair of output terminals 50 and 51.

The relay 31 includes a winding 52 connected between the terminals 50and 51 of the bridge 30, and a pair of contacts 54 under the control ofthe winding 52 and connected in series with the light 32 across thewinding 36 of the transformer.

The assembly 18 includes a connector element 55 having a cylindricalbase section 56 and a tubular section 57. The base section 56 isprovided with a bore 59 into which the end of the element section 11 isfitted. The section 56 is then welded or otherwise bonded to the sheath21 so as to provide a water and gas tight connection.

The tubular section 57 houses the transformer core section 20 and thewinding 19 and is provided with threads 60 on the outer surface thereof.

The assembly 17 includes a similar connector element 61 having acylindrical base section 62 provided with a bore 64 for receiving theend of the element section and having a tubular section 65 for housingthe transformer core section 20 and winding 19. The outer surface of thetubular section 65 is provided with a shoulder 66 and an annular nut 67having a flange 69 engaging the shoulder 66 and having internal threads70 for engaging the threads 60 on the assembly 18.

The core sections 20 are cylindrical members of ferro magnetic metal inwhich an annular recess 71 has been provided to form an innercylindrical pole piece 72 and an outer pole piece 74. The winding 19 iswound across the pole piece 72 and the remainder of the recess 71 isfilled with a mass of an insulating material such as epoxy resin to forma flat and completely sealed end surface.

In operation, the element sections 11 are joined together by threadingthe annular nuts 67 of the assemblies 17 onto the threads 60 of theassemblies 18. The pole pieces 72 and 74 of the mating assemblies 17 and18 are thereby pressed against each other as shown in FIG. 2 to formcomplete transformers interconnecting the sections 11. In the likemanner, the assembly 25 of the cable 13 is connected to the assembly 17on the end of the element 10.

Alternating current flows from the coil 35 of the transformer 27 throughthe bridge 29 between the input terminals 37 and 39.

The element 10 presents an impedance between the terminals 37 and 41which is proportional to the current drawn by the winding 19 of thetransformer half within the assembly 25. When the element 10 is atnormal ambient temperatures, the material 16 has a high resistance andthe current which flows through the wires 24 and 25 of the cable 13 isthat required to maintain the magnetic fields within the transformersassemblies. The impedance devices 42, 44, and 45 have impedance valuesproportional to each other and to the normal impedance between theterminals 37 and 41 such that no current flows between the outputterminals 40 and 41 when the element 10 is at normal ambienttemperatures.

When any portion of one of the element sections 11 is exposed to theabnormal temperature of a fire, the material 16 in that portion of thesection decreases sharply in resistance and provides a second currentpath. The current flowing into that section 11 from the winding 19 inthe assembly 17 now divides, part of it flowing through the material 16at the hot spot and part flowing to the winding 19 in the assembly 18 atthe other end of the section. The second current path draws additionalcurrent from the winding in the assembly 17 and this additional demandis transferred back through the transformer couplings to the winding inthe assembly 25. Thus, the impedance present between the terminals 37and 41 decreases and the bridge 29 is unbalanced. Current now flowsbetween the terminals 40 and 41 through the rectifying bridge 30 and therelay winding 52. The relay 31 is energized closing the contacts 54 andlighting the lamp 32.

From the foregoing, it will be seen that the present invention providesan improved temperature detecting apparatus in which the detectingelement sections are inductively coupled so as to eliminate from theelement section connectors all electrical contacts which could bebridged or corroded by moisture within the connector. All false alarmsdue to contamination of the connectors are thereby eliminated.

As various changes may be made in the form, construction and arrangementof the parts herein, without departing from the spirit and scope of theinvention and without sacrificing any of its advantages, it is to beunderstood that all matter herein is to be interpreted as illustrativeand not in any limiting sense.

What is claimed is:

1. A temperature detecting system including first and second elongatedelements each having first and second ends and comprising a pair ofparallel spaced electrical conductors extending between said ends, atleast said second element also including a mass of material between andin electrical contact with the conductors thereof, said material havingan electrical resistance which changes in value in response totemperature change to alter the resistance between the conductors, meansfor holding the first end of said second section closely adjacent to thesecond end of said first section, means in said first end of said secondsection and in said second end of said first section for inductivelycoupling the conductors in said first section to the conductors in saidsecond section, a source of varying potential electrical power coupledto said first end of said first section to be transmitted through saidinductive coupling means into said second section, means for detecting achange in the resistance of said material by monitoring the impedanceappearing at said first end of said first section, and means for givingan indication when said impedance reaches a predetermined value.

2. A system according to claim 1, wherein said inductive coupling meansin said first end of said second element section includes a half of atransformer having a winding connected across the pair of conductorstherein, said inductive coupling means in said second end of said firstsection includes a half of a transformer having a winding connectedacross the pair of conductors therein, and wherein said holding meanspositions said transformer halves to provide magnetic couplingtherebetween.

3. A system according to claim 2, wherein an inductive coupling means isprovided in said second end of said second element section and includesa half of a transformer having a winding connected across the pair ofconductors therein.

4. A system according to claim 2 wherein said transformer halves eachinclude a transformer core section, each of said windings are wound onsaid core section, and said core sections are placed in abuttingrelationship by said holding means to form a complete transformer.

5. A temperature detecting system including first and successivedetecting element sections each having first and second ends and beingpositioned in sequence so that the second end of each section isadjacent to the first end of the succeeding element section, each ofsaid sections comprising a pair of parallel spaced electrical conductorsextending between said ends thereof and a mass of material between andin electrical contact with the conductors thereof, said material havingan electrical resistance which Changes in value in response totemperature changes to alter the resistance between the conductors, asource of varying potential electrical power coupled to the first end ofsaid first element section to impress said potential across theconductors of said first section, means for providing a portion of atransformer in each of adjacent ends of consecutive element sections,said transformer portions in adjacent ends cooperating to provideinductive coupling between consecutive sections for impressing saidpotential across the conductors of said successive sections and meansfor detecting a decrease in resistance of said material in any of saidsections by monitoring the impedance appearing at said first end of saidfirst section.

6. A system according to claim 5, wherein each end of each of saidsections is provided with a half of a transformer including a windingconnected across the pair of conductors in the section.

References Cited UNITED STATES PATENTS 2,802,924 8/1957 Hebenstreit338-46 ROBERT L. RICHARDSON, Primary Examiner US. Cl. X.R. 323-69; 33826

